Present day rotor blades for helicopters are predominantly made of fiber composite materials. Since the rotor blades are deflected in various directions during operation of the helicopter, the blades are thereby subjected to high loads. A rotor blade of a bearingless rotor includes a so-called flexbeam or flexible spar, i.e. a structural element that is torsionally and flexibly soft, provided at the inboard end of the rotor blade adjoining the rotor head. The flexibility of this flexbeam enables the necessary deflection movements of the blade in the flapping direction, the lead-lag direction, and the torsional twisting direction to achieve pitch angle variations. Furthermore, the flexbeam carries and transmits the centrifugal force of the rotor blade into the rotor head.
The torsionally soft or flexible region of the flexbeam is located within a torsionally stiff control sleeve or torque tube, by means of which the control motions for achieving a pitch control of the blade are transmitted into the lift-generating airfoil blade portion of the rotor blade. In order that the flexbeam may be manufactured separately from the blade, or that the flexbeam may be replaced in the event of damage or the like, a separatable junction is typically provided between the flexbeam and the lift-generating airfoil portion of the rotor blade. Such a rotor blade is called a two-piece blade construction. The separatable junction also allows the rotor blade to be folded, for example by removing connectors or fasteners such as bolts or the like from the junction, whereby the rotor blade may be pivoted in the lead-lag direction into a folded position in which the rotor requires less space, so that the helicopter may be stored or received in a transportation vehicle such as an aircraft, a ship, or a ground vehicle for transporting the helicopter in a space-saving manner.
The control sleeve or torque tube is connected to a control system of the helicopter, so that control movements can be transmitted into the lift-generating airfoil blade via the control sleeve. Such a control movement especially refers to an angular deflection of the rotor blade about its torsional axis or central lengthwise axis, in order to change the pitch of the blade. In the rotating rotor blade, in connection with the pitch deflection, the blade also undergoes a pivoting or oscillating motion away from the nominal rotor blade lengthwise axis in a so-called lead-lag plane, i.e. a plane that is substantially identical to the rotation plane of the rotor blade.
In order to ensure an adequate ground and air resonance stability of the rotor, the lead-lag pivoting or oscillating movements of the rotor blade must be damped, for which dampers or damping elements are necessary. Such dampers or damping elements, which are also known as snubbers, typically comprise a multi-layered elastomeric composite including plural elastomeric layers, and usually also including metal layers for reasons of strength and stiffness. The required damping is provided by the periodic shearing strain and deformation of these elastomeric layers.
In previously known rotor blade constructions, the damping elements are arranged between the point at which the rotor blade is connected to the rotor head and the fictitious lead-lag hinge formed by the lead-lag flexing region. Moreover, for example in the disclosure of German Patent 35 26 470 and corresponding U.S. Pat. No. 4,690,616 (Hahn et al.), the damping elements are arranged outside of the control sleeve, and particularly on the outer wall of the control sleeve near the inboard edge thereof adjacent to the rotor head. The damping elements are located directly above and below the flexbeam. Thereby, the damping elements are arranged perpendicularly above and below the lead-lag plane of the rotor blade.
Due to the relatively large resulting spacing between the damping elements and the fictitious lead-lag hinge in the above prior art arrangement, this results in a relatively large deflection or deformation distance of the damping element layers as the blade undergoes the lead-lag oscillations, which in turn leads to the requirement of a relatively large structural height of each overall damping element in order to achieve a sufficient operating lifetime of the damping element. The relatively large structural height of the damping elements and their arrangement above and below the flexbeam have a negative influence on the aerodynamic characteristics of the blade arrangement. Furthermore, the external placement of the damping elements can also interfere with the free mobility of the control rods or rotor head covers or the like.
The conventional damping elements having a high structural height themselves also suffer disadvantages. Such damping elements consist of several stacked elastomeric layers, with stiffer layers interposed therebetween in order to take up the axial loads. Such dampers are relatively costly and complicated to manufacture, resulting in a rather high cost for the finished element. In order that damping losses do not arise, the mounting or attachment of the damping elements and the force transmission between the damping elements and the rotor structure must be carried out with tight tolerances, without play, and with a very stiff resulting structure. These factors also contribute to the complexity and cost of manufacturing and installing such conventional damping elements in the conventional arrangement.
German Patent Laying-open Publication 37 07 333 recognizes the problem that a damping of the lead-lag oscillation of the rotor blade is necessary due to possible unstable oscillations of the rotor blade especially during the run-up and the run-out of the rotor, and teaches how the damping problem can be solved by appropriate linkage connection of the control rod. Dampers are arranged on both sides of the root of a one-piece or integral rotor blade, wherein the root of the rotor blade is secured to the rotor head and forms a part of the flexbeam or flexible spar. The control rod is connected directly to a stiff connecting member interposed between the damping arrangement and the flexible spar.
Even though such an arrangement according to German Patent Laying-Open Publication 37 07 333 repositions the dampers from the outside surface of the control sleeve into the interior of the control sleeve and thereby closer to the area of the flexbeam root, there still exist relatively large displacements of the damping layers because the spacing between the damping elements and the fictitious lead-lag hinge still remains relatively large in such an arrangement. Once again, such a high displacement or high strain loading of the damping elements leads to the need of a relatively large structural height of the damping element in order to achieve an adequate operating life, because a thinner damping layer or fewer damping layers would not be able to durably provide the required total degree of displacement. Once again also, such dampers are complex and costly in their manufacturing, resulting in a high finished product cost. Basically, the arrangement suggested by German Patent Laying-Open Publication 37 07 333 simply achieves an improvement of the aerodynamic characteristics, by moving the damping elements from the outside to the inside of the control sleeve.
According to German Patent 29 27 263 and corresponding U.S. Pat. No. 4,244,677 (Noehren et al.), the damping elements are also arranged in the area of the root of the flexbeam within the control sleeve or housing, generally corresponding to the above described other conventional arrangements. German Patent Laying-Open Publication 37 34 592 also uses a damping arrangement generally located in the area of the root of the flexbeam, and provides the further improvement that the damping arrangement is positioned in the lead-lag plane. Nonetheless, relatively large deflections of the single damping element still arise and a relatively large structural height of the damping element is necessary, in view of the spacing between the damping element and the fictitious lead-lag hinge.